Conventionally, there is known an illumination system that turns on an illumination device by detecting the presence of a human body with a human detection sensor. A passive infrared radiation (PIR) sensor is generally used as the human detection sensor. The PIR sensor is of a passive type that detects the presence of a human body by sensing a change in the amount of received infrared ray caused by the movement of the human body within a detection area of the sensor.
However, a very small amount of the infrared rays is emitted from a human body. Therefore, if a human detection sensor is provided at the ceiling of a room for instance, the distance between the human detection sensor and the human body to be detected becomes too long. This results in reduced sensing accuracy.
Taking this into account, it may be thinkable to use, as the human detection sensor, an active sensor capable of highly accurately sensing a detection object even when the detection object is distant from the sensor. The active sensor is designed to, e.g., emit a wave toward the detection object and receive the reflected wave from the detection object. If the detection object is in motion, the frequency of the reflected wave makes change with respect to the transmitted wave by the Doppler effect. The active sensor detects the presence of the moving detection object (hereinafter referred to as a moving object) by detecting the frequency change. Using this detection principle, it is possible to detect a motor vehicle as well as a human body.
The amount of change in the observed frequency, i.e., the Doppler frequency fd, may be expressed by Eq. (1)
                                          f            d                    =                                                    2                ⁢                                                                  ⁢                                  f                  0                                            c                        ·            V                          ,                            Eq        .                                  ⁢                  (          1          )                    where f0 is the frequency of the emitted wave; V is the velocity of the moving object; and c is the speed of light. As can be seen in this equation, the Doppler frequency fd varies depending on the velocity V of the moving object.
There is known a moving object detection device that makes use of the above-noted characteristics to determine whether a moving object is a human being or not (see, e.g., Japanese Patent Laid-open Application No. 2007-127461). With this moving object detection device, the Doppler waveform extracted from the wave received by an active sensor is Fourier-transformed to eventually generate a spectrum waveform. If there exist two peak frequencies in the spectrum waveform, it is determined that they are attributable to the movement of the hands and the feet moving at different velocities, thus making a judgment that the moving object is a human being. In contrast, if there exists a single peak frequency, it is determined that a motor vehicle is moving at a constant speed, thus making a judgment that the moving object is a motor vehicle. The kind of the moving object is judged in this manner.
With the moving object detection device mentioned above, however, it is impossible to detect the velocity of the moving object. In an illumination system, there is a need to detect the travel velocity of a moving object with an active sensor and to perform illumination control according to the travel velocity thus detected. Since, however, the travel velocity of a moving object cannot be detected even when employing the configuration of the aforementioned moving object detection device, it is difficult to perform illumination control according to the travel velocity.